Porous separation membranes that take a form of a hollow fiber or a flat membrane are used as semipermeable membranes for removing fine particles. The porous separation membranes utilize selective permeability of semipermeable membranes, and a substance is filtered and separated therethrough to remove fine particles that cannot pass through the semipermeable membranes. Since the diffusing rate of a substance that passes through a semipermeable membrane is low, a porous separation membrane is required to have a small thickness; however, thin separation membranes are prone to breaking.
A semipermeable membrane, which has been made porous by stretching a fluororesin thin membrane fixed onto a support, has been proposed as a porous separation membrane that has a small thickness and is less prone to breaking (refer to Japanese Unexamined Patent Application Publication No. 2010-94579). This porous separation membrane is formed by stretching a resin coating, which is stacked on a stretchable support, at a temperature lower than 30° C. so as to suppress occurrence of breaking of the separation membrane.
According to the porous separation membrane of the related art described above, the fluororesin thin membrane that functions as a semipermeable membrane is produced by applying a fluororesin dispersion, drying the applied fluororesin dispersion, and sintering the dried fluororesin dispersion. Moreover, the semipermeable membrane that can be produced by this method has an average thickness of less than 20 μm and a pore size of 45 nm or less.